This invention relates to a droplet generator for a continuous stream ink jet print head.
A conventional droplet generator has an elongate cavity for containing the ink and nozzle orifices communicating from within to without the cavity for passing ink from the cavity to form jets. The nozzle orifices extend along the length of the cavity. An actuator means vibrates the ink in the cavity such that each jet ideally breaks up into ink droplets at the same predetermined distance from its respective nozzle orifice. The actuator means is disposed on the opposite side of the cavity to the nozzle orifices, and a surrounding wall of the cavity extends between the actuator means and the nozzle orifices for containing the ink between the actuator and the nozzle orifices.
In one known generator as described in the previous paragraph, the nozzle orifices are formed in a stainless steel foil sheet which is bonded to a stainless steel plate which is bolted by means of stainless steel bolts to a stainless steel manifold.
A problem with this known generator is that vibration of the various constituent parts of the generator is communicated to the ink cavity, and hence may perturb the uniform break up of the jets along the length of the cavity. A further problem with this known generator is the time consuming adjustment of the tightness of the bolts during set up of the generator. This adjustment is required to minimize vibration of the foil sheet, and thereby minimize variation in jet break up distance from the generator along the length of the cavity. In addition, tightening the bolts can distort the plate to which the foil sheet is secured, hence undesirably stretching the foil sheet. The known generator is also sensitive to mechanical and thermal stresses in its structure.
A droplet generator of the kind described in the preceding paragraph is disclosed in WO-A-9006850.
According to the present invention there is provided a droplet generator for a continuous stream ink jet print head comprising: an elongate cavity for containing the ink; nozzle orifices communicating from within to outside of said cavity for passing ink from the cavity to form jets, said nozzle orifices extending along the length of said cavity; actuator means for vibrating the ink in said cavity such that each said jet breaks up into ink droplets at the same predetermined distance from its respective nozzle orifice, said actuator means being disposed on the opposite side of said cavity to said nozzle orifices; and extending between said actuator means and said nozzle orifices a surrounding wall of said cavity for containing the ink between said actuator means and said nozzle orifices, characterised in that said surrounding wall comprises inner and outer constituent walls, the resistive component of the wave impedance of said outer wall being such that the outer wall passively damps vibration of the inner wall by dissipating its vibration, the reactive component of the wave impedance of said outer wall being such that the outer wall actively restrains vibration of the inner wall, said outer wall thereby ensuring that each said jet breaks up into ink droplets at said same predetermined distance from its respective nozzle orifice.
Preferably, said actuator means is held in said generator by the outer wall of said surrounding wall.
Preferably, said cavity has a xe2x80x98Vxe2x80x99 cross section defined by the interior faces of said inner wall, and said nozzle orifices extend along the open apex of said xe2x80x98Vxe2x80x99 cross section cavity. The nozzle orifices may be formed in a laminar sheet which is secured to said inner wall such that the nozzle orifices extend along said open apex of the xe2x80x98Vxe2x80x99 cross section cavity.
Preferably, said actuator means overlaps the ends and sides of said cavity where it addresses the cavity.
Preferably, said outer wall is a single piece moulded construction. Alternatively, said outer wall may be constructed from at least two parts secured together.
Preferably, said actuator means and said inner wall are push fitted in said outer wall.
Preferably, said inner wall is metal and said outer wall is plastic.
Preferably, said actuator means is designed to be resonant in operation of the generator, whereas said cavity is designed not to be.